In Ser. No. 08/211,983 a process is described for the reduction of the chlorine content of polychlorinated hydrocarbons such as, for example, polychlorinated benzene and polychlorinated biphenyls. The process utilizes a reagent comprising (i) at least one complex of a transition metal of group 4 or 5 with a multidentate or unidentate organic or inorganic ligand and (ii) a reducing agent such as a hydridoborate. Typical of the first component is bis-(.eta..sup.5 -cyclopentadienyl)titanium dichloride (titanocene dichloride) while sodium tetrahydridoborate is representative of the latter.
Various methods are employed in reducing nitrogen-containing substituents on aliphatic and aromatic compounds to amines. For example, the reduction of nitro substituents to amines involves reducing agents such as iron, zinc, or tin with acid; hydrogen with a catalyst such as platinum, palladium, or nickel; hydrogen with a catalyst; and sulfides.
Nitroso substituents and hydroxylamines can be reduced to amines by the same reagents which reduce nitro compounds. N-Nitroso compounds are similarly reduced to hydrazines. Azo, azoxy, and hydrazo substituents can be reduced to amines using metals, notably zinc, and acids, and sodium hydrosulfate as reducing agents. Diborane reduces azo substituents to amines.
Nitro substituents have been reduced to intermediates within the sequence, such as azoxy, with sodium arsenite, sodium ethoxide, glucose, and potassium borofluoride. The most common reducing agents for reducing nitro substituents to azo compounds are lithium aluminum hydride, and zinc and alkali. Other intermediate substituents within the sequence, such as nitroso, are reduced to azo compounds with lithium aluminum hydride as well. Zinc and sodium are the most common agents used in reducing nitro substituents to hydrazo compounds. Nitro substituents have also been reduced to hydrazo compounds electrolytically, or with lithium aluminum hydride mixed with a metal chloride such as titanium tetrachloride or vanadium trichloride, or hydrazine hydrate and Raney nickel.
On aliphatic compounds, lithium aluminum hydride reduces nitro substituents to amines, but nitro substituents on aromatic compounds are reduced to azo compounds. Lithium aluminum hydride does not generally reduce azo compounds (these are the products from lithium aluminum hydride reduction of nitro compounds), but these substituents can be reduced to hydrazo compounds by catalytic hydrogenation.
Most metal hydrides, such as sodium borohydride, reduce nitro substituents on aromatic compounds to azo and azoxy compounds, leaving the aromatic ring intact.